1. Field of the Invention
The present invention relates to a method for producing an electron source, an electron source produced by such method, a method for producing an image forming apparatus and an image forming apparatus produced by such method.
2. Related Background Art
The conventional electron emission devices are classified into a hot electron emission device and a cold cathode electron emission device. The cold cathode electron emission device includes a field emission type (hereinafter called “FE type”), a metal/insulating layer/metal type (hereinafter called “MIM” type) and a surface conduction electron emission device.
The FE type element has been disclosed for example by W. P. Dyke and W. W. Dolan, “Field Emission”, Advance in Electron Physics, 8, 89(1956) and C. A. Spindt, “Physical properties of thin-film field emission cathodes with molybdenum cones”, J. Appl. Phys., 47, 5248(1976).
Also the MIM type element has been disclosed for example by C. A. Mead, “Operation of tunnel-emission devices”, J. Appl. Phys., 32, 646(1961).
Also the surface conduction electron emission device is disclosed for example by M. I. Elinson, Radio Eng. Electron Phys., 10, 1290(1965).
The surface conduction electron emission device utilizes a phenomenon of inducing electron emission by giving a current, on a thin film of a small area formed on an insulating substrate, parallel to the film. The surface conduction electron emission device has been reported in various structures such as a structure with a SnO2 film reported by Elinson mentioned above, one with Au film (G. Dittmer, Thin Solid Films, 9, 317(1972), one with In2O3/SnO, film (M. Hartwell and C. G. Fonstad, IEEE Trans. ED Conf., 519(1957), and one with carbon film (H. Araki et al., Shinku (Vacuum), 26, No. 1, 22(1983).
As a typical example of the surface conduction electron emission devices, the configuration of the above-mentioned device by M. Hartwell is schematically shown in FIG. 16, wherein shown are a substrate 1901, a conductive film 1904 consisting for example of a metal oxide film formed by sputtering in an H-shaped pattern, and an electron emission part 1905 formed by a current passing process, called electroforming to be explained later. In the drawing, the element distance L is selected as 0.5 to 1 mm, and W′ is selected as 0.1 mm.
In such electron conduction electron emission device, the electron emitting portion 1905 is generally formed, prior to the electron emission, by subjecting the conductive film 1904 to a current passing treatment which is called electroforming. More specifically, the electroforming is a process of applying, across the conductive film 1904, a DC voltage or a very slowing increasing voltage for example at a rate of 1 V/min, thereby causing local destruction, deformation or denaturing of the conductive film 1904 with a structure change therein, thus forming the electron emitting portion 1905 of a high electrical resistance. In the electron emitting portion 1905, cracks are formed in a part of the conductive film 1904 and the electron emission takes place from the vicinity of such cracks.
The surface conduction electron emission device subjected to the above-mentioned electroforming is capable of emitting electrons from the electron emitting portion 1905 by a current passing in the element under a voltage application thereto. Also the present application proposes a deposition process of significantly varying the current in the above-mentioned conductive film 1904 (hereinafter called “device current”) and the current emitting in vacuum space (hereinafter called “electron emission current”) (Japanese Patent Application Laid-open No. 7-235255).
Such surface conduction electron emission device, being simple in configuration, has an advantage that there can be easily prepared an electron source consisting of an array of a plurality of elements over a large area. Such feature is being investigated in various applications, such as use in an image forming apparatus such as a light-emitting thin image display apparatus.
With respect to the electron emitting characteristics, a further improvement in uniformity is being desired in order that the image forming apparatus utilizing such electron emission device can stably provide a bright displayed image. The efficiency of such element can be represented by the ratio of the device current and the electron emission current, and there is being desired an electron emission device with a smaller device current and a larger emission current. If the multiple electron emission devices constituting an electron source can be made uniform in the electron emitting characteristics, there can be realized an image forming apparatus utilizing a fluorescent material as the image forming member, for example a flat television unit, of a higher brightness and higher quality.
The present inventors have conducted research on the electron source consisting of an array of multiple surface conduction electron emission devices and the image forming apparatus utilizing such electron source, including the electron source based on the electrical wiring method shown in FIG. 5.
More specifically, the electron source is constituted by arranging a plurality of surface conduction electron emission devices in two-dimensional manner and wiring these elements in a matrix manner as illustrated. In FIG. 5 there are shown surface conduction electron emission devices 504 represented in schematic manner, row wirings 502 and column wirings 503. The wiring method shown in FIG. 5 is called simple matrix wiring.
In the electron source constituted by simple matrix wiring of multiple surface conduction electron emission devices as shown in FIG. 5, suitable electrical signals are applied to the row wiring 502 and the column wiring 503 in order to output a desired electron beam. For example, for driving the surface conduction electron emission devices of an arbitrary row in the matrix, a selection voltage Vs is applied to the row wiring 502 of a selected row while a voltage Vns is applied to the row wirings 502 of the non-selected rows. In synchronization, a drive voltage Ve for outputting the electron beam is applied to the column wiring 503.
In this method, if the voltage drop resulting from the resistance in the wirings is disregarded, the surface conduction electron emission devices of the selected row receive a voltage Ve−Vs, while those of the non-selected row receive a voltage Ve−Vns, and the suitable selection of Ve, Vs and Vns should cause emission of the electron beam of a desired intensity from the surface conduction electron emission devices of the selected row only while the application of respectively different drive voltages Ve to the column wirings should cause emission of the electron beams of different intensities from the respective elements of the selected row.
Also, as the surface conduction electron emission device has a high response speed, the duration of electron beam output should be varied by varying the duration of application of the drive voltage Ve. Consequently, the electron source consisting of simple matrix wiring of multiple surface conduction electron emission devices has possibility of various applications, and can be advantageously utilized as the electron source for the image display apparatus under the application of suitable electrical signals corresponding to the image information.
Further, the present inventors have conducted extensive investigations for further increasing the current emitted from the surface conduction electron emission device into vacuum space (hereinafter called electron emission current Ie) and improving the efficiency of such current, and have found that the electron emission current Ie in vacuum can be increased by adding a new step, called deposition process step, thereby forming deposition in the cracks of the conductive film.
The deposition process step is applied to the element after the forming process and is to repeat pulse application of a predetermined voltage under vacuum of 1×10−2 to 1×10−3 Pa to cause deposition from substances present in the atmosphere, thereby significantly increasing the emission current Ie.
However, for example in case of preparing an electron source consisting of multiple surface conduction electron emission devices connected in a simple matrix of m rows×n columns, and if the 1st to m-th rows are subjected to such deposition process in succession for example with a process time of 30 minutes per row, there will be required an enormous process time of 30×m minutes and the amount of substances in the atmosphere will vary in such prolonged period, whereby the deposition process cannot be applied under a same condition for all the lines and the uniform electron emission characteristics cannot be obtained. In consideration of the foregoing, the present applicant has proposed, in the Japanese Patent Application Laid-open No. 9-134666, a method for producing the electron source including the deposition step in which the multiple electron emission devices are divided into plural groups and the voltage application is conducted in succession to such groups thereby causing deposition in the electron emission portions of the plural electron emission devices.